The present invention relates to code-division multiple access (CDMA) radiotelephone communications.
CDMA is a method of spread spectrum digital communication in which a plurality of transmission channels are created by using spreading sequences for each channel that modulate the information bits to be transmitted. The spreading sequences operate at a chip rate higher than the data bit rate in order to achieve spectrum spreading of the radio signal. Their self- and cross-correlation properties are adapted to enable the various channels to be multiplexed: they are generally pseudorandom sequences that are mutually orthogonal or quasi-orthogonal, taking chip values of -1 or +1.
The use of CDMA in the field of cellular radiotelephony is described in chapter I of the work "Mobile radio communications" by Raymond Steele, Pentech Press, London 1992, and also in the article "On the system design aspects of code division multiple access (CDMA) applied to digital cellular and personal communications networks" by A. Salmasi and K. S. Gilhousen, Proc. of the 41st IEEE Vehicular Technology Conference, St. Louis, Mo., 19-22 mai 1991. The multiplexed transmission channels are formed at the base station of each cell in the network. Each mobile station situated within the cell uses a special spreading sequence to recover, from the overall radio signal transmitted by the base station, the data bits that are adressed thereto.
In the system described in the above publications, the various spreading sequences are obtained from a common reference sequence having a chip rate of 1.2288 MHz, and period of 32 768 chips. Since the radio modulation is quadrature phase shift keying, the reference sequence includes an in-phase component and a quadrature component. Sixty-four transmission channels are formed in the base station by combining the reference sequence with each of the sixty-four Walsh codes of length 64. The channel defined by Walsh code W.sub.0, which comprises nothing but 1, is a pilot channel over which no data bits are sent. The pilot channel does transmit the reference sequence synchronously with the set of spreading sequences. The mobile stations have a priori knowledge of the values of the reference sequence such that, on receiving the pilot channel, they can synchronize themselves with the base station to receive the data bits that are respectively addressed to each of them.
Whether or not use is made of the Walsh code technique, it is always useful to form a pilot channel enabling the mobile channels to synchronize themselves. The pilot channel carries a reference pseudo-random periodic sequence having the same chip rate as the spreading sequences and synchronized therewith.
Various methods exist for obtaining the desired synchronization, all of which make use of the advantageous self- and cross-correlation properties of pseudo-random sequences. They are based on calculating the cross-correlation between the sequence received on the pilot channel and a sequence tested by the mobile station, given that the results of such an operation will always be low except when the sequences are synchronized and identical.
A first synchronization method uses matched filters, i.e. filters whose coefficients are equal to the samples of the tested sequence. The result of such filtering is thus directly the value of the looked-for cross-correlation. Since the output rate of this filter is the same as the input rate, a correlation peak is detected when the mobile station is synchronized, and as a result the mean synchronization time is relatively short.
A second method makes use of correlators that apply the same principle. The received signal is multiplied by the tested sequence and integration over a plurality of samples makes it possible to detect a correlation peak, if any. When there is no correlation peak, then the operation is reiterated, either with the same sequence subjected to a time offset, or else with a different sequence. The mean time to obtaining synchronization is significantly longer than it is with the first method.
Both of those two methods suffer from the drawback of significantly degraded performance in the event of frequency deviation occurring in the received radio signal. Under such circumstances, it is no longer possible to associate a correlation peak with genuine synchronization. Frequency deviation may be due to the Doppler effect, to Rayleigh fading, or to differences in the characteristics of the local oscillators of the stations in communication. To cope with this problem, the correlations must be performed by making an assumption about the value of the frequency deviation to which the received signal has been subjected, and by using a battery of matched filters whose respective frequencies correspond to the various different possible values of deviation. The results of the various different filterings are compared and the largest correlation peak is used as a basis for determining which filter can be used to obtain the looked-for time and frequency synchronization. Such a solution is not optimal in terms of performance. In addition, it significantly increases the complexity of the receiver.
An object of the present invention is to remedy the above difficulties, by proposing a time synchronization method that is not very sensitive to possible frequency deviation in the radio signal.